The present invention relates to a method of air-fuel ratio control in which the air-fuel ratio is controlled to a level where the fuel consumption rate is the most suitable under the normal running conditions, while the air-fuel ratio is controlled by feedback to a level associated with the highest output under the throttle valve condition of high degree of opening.
The air-fuel ratio of an internal combustion engine (hereinafter referred to as the engine) is set to a stoichiometric air-fuel ratio or a lean air-fuel ratio placing primary emphasis on the fuel consumption rate under normal running condition, At the time of acceleration or ascending a slope when the thorottle valve opens widely, on the other hand, the air-fuel ratio is set to about 13 associated with the highest output. When the engine is idling, the air-fuel ratio is set to a level taking the rotational stability of the engine, etc. into consideration.
First, description will be made of the air-fuel control under the normal running conditions for control of fuel consumption. The prior art carburator control is of open loop control type in which variations of different engines, secular change or change in lapse of time of an engine, and production variations of the carburetor have caused some loss of the fuel efficiency. In an electronic fuel injection system in which the amount of intake air into the engine is measured directly by an intake air amount sensor, the required quantity of fuel supply is computed by computer or the like and the fuel is supplied into the intake pipe by an electromagnetic valve according to the computation, and a closed loop control system is employed for determining the direction of the stoichiometric air-fuel ratio of about 15 on the basis of the output of an oxygen sensor provided in the exhaust pipe thereby to correct the amount of fuel supply. Also, a carburetor of closed loop control type finds applications in which the direction of the stoichiometric air-fuel ratio is determined by the output of the oxygen concentration sensor thereby to correct the air bleed quantity. These closed loop control systems, though effective in correcting the variations of air-fuel ratio, still cause a fuel loss due to the fact that the stoichiometric air-fuel ratio is not an air-fuel ratio of minimum fuel consumption.
A method of controlling the fuel consumption rate to the best level by eliminating these disadvantages is disclosed in U.S. Pat. No. 4,026,251. According to this patent, the air bypassing the carburetor is caused to dither, that is, switched at regular intervals of time between richer and leaner sides of air-fuel ratio, so that the direction of air-fuel ratio for improvement of fuel consumption rate is determined and the opening of an auxiliary air valve bypassing the carburetor is controlled accordingly thereby to correct the air-fuel ratio. In this device, the engine is operated once each at two points of reference air-fuel ratio provided on relatively rich and lean sides, and the engine speed N.sub.eR at the point of the rich air-fuel ratio is compared with the engine speed N.sub.eL at the point of lean air-fuel ratio. And if N.sub.eR is larger than N.sub.eL (N.sub.eR &gt;N.sub.eL), the bypass air is reduced; while if N.sub.eR is smaller than N.sub.eL (N.sub.eR &lt;N.sub.eL), the bypass air is increased.
In the case where a change of, say, the output of the engine is determined by the engine speed, however, it is impossible by this method to determine whether the change of the engine speed is caused by the change of the air-fuel ratio or by an external factor such as accelerator pedal operation, an ascending slope or descending slope etc., thus often resulting in the engine being controlled toward a deteriorated instead of improved fuel efficiency, in view of the fact that the engine speed depends on various factors. Also, the air-fuel ratio is corrected by changing the air amount bypassing the carburetor, so that an auxiliary air valve for changing the area with high accuracy is required but not yet commercially realized. Further, in spite of the fact that an air-fuel ratio for the highest output is required to be set at the time of acceleration or ascending the slope when the throttle valve widely opens, a method has not yet been realized in which the air-fuel ratio is controlled to become a value which causes the best fuel consumption rate under normal running conditions and to become a value which causes the highest output at the time of acceleration or ascending a slope.